The present invention generally relates to an electrostatic recording apparatus and more particularly, to an electrostatic recording apparatus employing a single-faced and double-sided control type (also referred to as "a side face control type") recording head.
Conventionally, in electrostatic recording apparatuses of the above described type, it has been so arranged as shown in FIGS. 1 and 2 that a single-faced and double-sided control type recording head (hereinbelow, referred to as "a stylus head") 5 is provided so as to confront an electrically insulative elastic roller 4 such that an electrostatic recording paper P including a base sheet 1, a conductive layer 2 provided on the base sheet 1, and a dielectric layer 3 provided on the conductive layer 2 can be interposed between the stylus head 5 and the elastic roller 4 with the dielectric layer 3 facing the stylus head 5. The stylus head 5 includes a plurality of needle-like recording electrodes 6 and a plurality of controlling electrodes 7 and 8. The recording electrodes 6 are arranged at right angles to the transport direction of the electrostatic recording paper P. It is to be noted that the electrostatic recording paper P is transported in the direction of the arrow A in FIGS. 1 and 2. The controlling electrodes 7 are disposed in parallel with the recording electrodes 6 and at the upstream side of the recording electrodes 6 with respect to the transport direction of the electrostatic recording paper P while the controlling electrodes 8 are disposed in parallel with the recording electrodes 6 and at the downstream side of the controlling electrodes 6 with respect to the transport direction of the electrostatic recording paper P. Furthermore, a grounding roll 9 for removing unnecessary electric charges from the electrostatic recording paper P is provided at the upstream side of the controlling electrodes 7.
In order to simplify the drive circuit of the stylus head 5, it has been so arranged that the recording electrodes 6 are divided into groups each containing a predetermined number of the recording electrodes 6, and a voltage of -VH (or +VH) is applied to each group of the recording electrodes 6. After a width L3 of each of the controlling electrodes 7 and 8, measured at right angles to the transport direction of the electrostatic recording paper P is set at a value so as to contain a proper number of the recording electrodes 6 therein, a voltage of +VH (or -VH) is applied to the controlling electrodes 7 and 8 such that a matrix circuit is formed. In the matrix circuit, it is so arranged that in the case where both the recording electrodes 6 and the controlling electrodes 7 and 8 are set to the ON state for recording on portions of the electrostatic recording paper P at a recording position W of the stylus head 5, a sufficient amount of electric charge enabling adherence of toner to the electrostatic recording paper P is accumulated on the recording portions of the electrostatic recording paper P. Furthermore, in the matrix circuit, it is so arranged that a sufficient amount of electric charge enabling adherence of toner to the electrostatic recording paper P is not accumulated on the electrostatic recording paper P in the case where only one or the other of the recording electrodes 6 and the controlling electrodes 7 and 8 is set to the ON state. The width L3 of each of the controlling electrodes 7 and 8, measured at right angles to the transport direction of the electrostatic recording paper P, is set, for example, at 3.5 mm approximately in the case where 32 controlling electrodes 6 are contained in the width L3 in a stylus head having a resolving power of 8 lines per mm. Meanwhile, the clearance L4 between adjoining controlling electrodes 7 and also between adjoining controlling electrodes 8 is set at 0.5 mm approximately. It should be noted that the length L1 of each of the controlling electrodes 7 and the length L2 of each of the controlling electrodes 8, each measured in the transport direction of the electrostatic recording paper P, are equal to each other and are set at 4 to 6 mm. Meanwhile, the distance l1 between the controlling electrodes 7 and the recording electrodes 6 and the distance l2 between the recording the electrodes 6 and the controlling electrodes 8, each measured in the transport direction of the electrostatic recording paper P is set at 0.5 mm approximately.
In order to raise the recording density of an image formed by the stylus head 5 of the above described type, voltages applied to the recording electrodes 6 and the controlling electrodes 7 and 8 are required to be increased since the time period for applying the voltages to the recording electrodes 6 and the controlling electrodes 7 and 8 cannot be lengthened very much in connection with a high recording speed of the stylus head 5. Thus, increase of the voltages applied to the recording electrodes 6 and the controlling electrodes 7 and 8 promotes accumulation of electric charge on portions of the electrostatic recording paper P, which portions are not recorded on, so that toner is caused to adhere, through development, to the portions on which no recording is desired and thus, so-called ghost images are formed, thereby resulting in deterioration the image quality of the electrostatic recording.
Hereinbelow, the mechanism of formation of ghost images will be described in detail.
Each time the controlling electrodes 7 are scanned, the voltage of +VH is applied to portions of the electrostatic recording paper P, with the portions being disposed immediately below the controlling electrodes 7 and thus, negative electric charge is accumulated on the portions. Although the negative electric charge accumulated on the portions is discharged therefrom when the controlling electrodes 7 reach a zero potential in the case where the dielectric layer 3 of the electrostatic recording paper P is in contact with the controlling electrodes 7, the negative electric charge is not readily discharged from the portions since a clearance of 5 to 10 .mu.m is actually formed between the dielectric layer 3 and the controlling electrodes 7. Although the grounding roll 9 provided at the upstream side of the recording position W is also adapted to discharge the accumulated negative electric charge, the grounding roll 9 loses its effect in the case where the resistance of the base sheet 1 is subjected to change due to ambient conditions such as temperature, humidity, etc. Thus, it may be understood that the negative electric charge is left on a part of the electrostatic recording paper P, to which part no voltage is applied from the recording electrodes 6, thereby resulting in formation of the ghost images in some kinds of toner. Furthermore, it is supposed that the controlling electrodes 7 each having the width L1 as large as 4 to 6 mm are likely to produce the ghost images.
More specifically, the mechanism of formation of the ghost images will be described with reference to FIGS. 3 and 4, hereinbelow.
It is to be noted here that FIG. 3 schematically illustrates a principle of the conventional electrostatic recording apparatus of FIG. 1, and FIG. 4 shows an equivalent circuit thereof. In FIG. 3, the recording electrodes 6 and the controlling electrodes 7 and 8 are spaced away from the dielectric layer 3. Thus, electrostatic capacities B1 and B3 are, respectively, produced between the recording electrodes 6 and the dielectric layer 3 and between the conductive layer 2 and each of the controlling electrodes 7 and 8 with the dielectric layer 3 having an electrostatic capacity B2. As shown in FIG. 4, the electrostatic recording paper P constitutes a resistor R1 between the controlling electrodes 8 and the recording electrodes 6, a resistor R2 between the recording electrodes 6 and the controlling electrodes 7 and a resistor R3 between the controlling electrodes 7 and the grounding roll 9. Capacitors C1 and C2 have the electrostatic capacities B1 and B2, respectively while each of the capacitors C3 and C4 has the electrostatic capacity B3. It is to be noted that a charging point is disposed between the capacitor C1 and the capacitor C2. A power source V' having the voltage of +VH is connected to ground on one hand and is, on the other hand, connected to the capacitor C3 leading to the resistor R3 connected to ground. The power source V' is also connected to the capacitor C4 and the capacitor C4 is, in turn, connected, through the resistor R1, to the resistor R2 leading to the resistor R3. A power source V having the voltage of -VH is connected to ground on one hand and is, on the other hand, connected, via the capacitor C1, to the capacitor C2 leading to the junction of the resistor R1 and the resistor R2. Thus the amount of electricity charged on the dielectric layer 3 varies with the electrostatic capacity B3 in the case where the electrostatic capacities B1 and B2 are constant due to capacitive coupling therebetween. Meanwhile, the electrostatic capacity B3 is proportional to the area of the face of each of the controlling electrodes 7 and 8, which face confronts the electrostatic recording paper P. Since it is so arranged that upon transport of the electrostatic recording paper P, the recording voltage of a polarity opposite to that of the voltage applied to the controlling electrodes 7 is applied, a matrix drive, to the recording portions of the electrostatic recording paper P by the recording electrodes 6, on which recording portions electric charge has been accumulated by the controlling electrodes 7 disposed at the upstream side of the recording electrodes 6, it is supposed that slight electric discharge takes place at portions of the electrostatic recording paper P, other than the recording portions, thereby resulting in formation of the ghost images. Accordingly, it can be concluded that the area of the face of each of the controlling electrodes 7 disposed at the upstream side of the recording electrodes 6 should be restricted to a minimum so as to decrease the electrostatic capacity B3 of the capacitor C3 such that accumulation of electric charge on the electrostatic recording paper P at the upstream side of the recording electrodes 6 is minimized at the face confronting the electrostatic recording paper P.
Conventionally, a number of countermeasures have been taken against formation of the ghost images, etc.
For example, Japanese Laid Open Patent Application No. 4128/1979 (Tokkaisho 54-4128) has proposed a method for preventing formation of the ghost images, in which clamping electrodes are provided on a part of the elastic roller 4 such that electric charge accumulated on the electrostatic recording paper P, except for electric charge accumulated on the recording portions thereof, is discharged to ground through the clamping electrodes. However, this method has such inconveniences that the mechanism and control circuit become complicated, the apparatus becomes large in size and the production cost of the apparatus is raised.
Japanese Laid Open Patent Application No. 1626/1979 (Tokkaisho 54-1626) has disclosed a method for preventing formation of the ghost images, in which a clamping electrode for grounding is provided adjacent to the controlling electrodes 7 disposed at the upstream side of the recording electrodes 6 with respect to the transport direction of the electrostatic recording paper P such that electric charge accumulated on the electrostatic recording paper P, except for electric charge accumulated on the recording portions thereof, is discharged to ground through the clamping electrode in the same manner as the above described method. However, this method has such a disadvantage that formation of images becomes unstable depending on the mounting accuracy of the clamping electrode. Furthermore, in this method, since in the same manner as the above described method, the clamping electrode for grounding is disposed adjacent to the recording electrodes 6 and the controlling electrodes 7 and 8 to which high voltages are applied, the apparatus presents a great danger to an operator thereof, so that it becomes necessary to provide a mechanism for preventing the recording electrodes 6 and the controlling electrodes 7 and 8 from being set to the ON state in the case of absence of the electrostatic recording paper P.
Furthermore, Japanese Laid Open Patent Application No. 34831/1979 (Tokkaisho 54-34831) has proposed a method for forming recording images of high density at a low recording voltage and for preventing formation of the ghost images, in which the voltage applied to the controlling electrodes 8 disposed at the downstream side of the recording electrodes 6 with respect to the transport direction of the electrostatic recording paper P is made lower than the voltage applied to the controlling electrodes 7 disposed at the upstream side of the recording electrodes 6 with respect to the transport direction of the electrostatic recording paper P such that electric charge is prevented from disappearing from the electrostatic recording paper P through neutralization of the surface potential of the electrostatic recording paper P at the time when the recording face of the electrostatic recording paper P is transported immediately below the controlling electrodes 8 disposed at the downstream side of the recording electrodes 6. However, this method involves such problems that the power source circuit and control circuit become complicated and the electrostatic recording apparatus becomes large in size and high in production cost.
Moreover, there have been proposed various methods in which timing of application of voltages to the controlling electrodes 7 and 8 is controlled in a sophisticated manner, the controlling electrodes 7 and 8 are formed into complicated shapes, an electrostatic recording paper adapted to make it difficult to form the ghost images thereon is utilized, etc. However, these known methods have been disadvantageous in that the electrostatic recording apparatus becomes large in size and high in production cost, the electrostatic recording paper becomes expensive, etc.
Meanwhile, conventionally, in electrostatic recording apparatuses, it has been so arranged as shown in FIG. 5 that the elastic roller 4 is brought into pressure contact with the recording electrodes 6 and the controlling electrodes 7 and 8. Namely, a rotary shaft 17 for the elastic roller 4 is supported by a bearing 20 provided on a frame 18 and a bearing 21 provided on a frame 19. Furthermore, pins 22 and 23 are secured to the frames 18 and 19, respectively and a tension spring 24 having one end attached to the pin 22 and a tension spring 25 having one end attached to the pin 23 are wound around the bearings 20 and 21, respectively. Thus, the elastic roller 4 is brought into pressure contact with the stylus head 5 such that the urging force of the tension springs 24 and 25 is balanced with a reactive elastic force of the elastic roller 4 and the electrostatic recording paper P is transported through a driving force from a gear 26 mounted on one end of the rotary shaft 17 adjacent to the bearing 20. However, the prior art electrostatic recording apparatuses are readily subjected to such inconveniences that an image has an unformed part, ghost images are formed and the background of the image becomes stained since the elastic roller 4 is not brought into pressure contact with the stylus head 5 uniformly, namely the elastic roller 4 is not positioned accurately with respect to the stylus head 5.